Backup your original Hard Drive
Phoem's Fast DD Method
This will allow you to make perfect backups of your AppleTV system partitions in case you need to restore them to a virgin state.
First make copies of the original AppleTV partitions that we care about (everything except for the media partition, basically)
These instructions assume /dev/disk2 is your AppleTV drive.
Lets copy the partitions onto your local drive:
dd if=/dev/disk2 of=Partition.img bs=1024k count=100
dd if=/dev/disk2s1 of=EFI.img
dd if=/dev/disk2s2 of=OS.img
dd if=/dev/disk2s3 of=OSBoot.img
Now lets put it back in place onto the new drive:
dd if=Partition.img of=/dev/disk2 bs=1024k count=10
dd if=EFI.img of=/dev/disk2s1
dd if=OS.img of=/dev/disk2s2
dd if=OSBoot.img of=/dev/disk2s3
Now use iPartition demo to find out the size if you were to grow the partition completely; using up the free space.
Execute: gpt remove -i 4 /dev/disk2
Unmount the drives.
Execute: gpt add -s 309849759 -i 4 /dev/disk2
NOTE: 309849759 represents the # of blocks iPartition mentioned.
Finally, open diskutility and format just the media partition. (erase)